The present invention relates to an air switch and palm guide for guiding the high speed fabrics which are used to carry the paper web through papermaking machinery.
In the papermaking process, thousands of gallons per minute of liquid pulp or xe2x80x9cstockxe2x80x9d is poured out of the head box over the forming board and on to the fourdrineer fabric or xe2x80x9cwirexe2x80x9d. At this point, paper is in liquid form, about 98% water and 2% fiber, filler, and chemicals. The fiber is usually made up from virgin ground wood and or recycled paper. The filler is usually clay and/or pulverized calcium carbonate mined from quarries.
A combination of chemicals are added to the water, fiber, and filler to give the paper certain desired properties. This liquid mixture is commonly referred to as the furnish. The mixture or exact recipe of the furnish will vary with different paper grades and types.
A typical wire can be a 140xe2x80x2 long loop. The term xe2x80x9cwirexe2x80x9d predates the invention of plastic type monofilament fourdrineer fabrics. Originally, the fourdrineer fabric was actually made of very fine strands of brass or bronze. The fourdineer or wire table is typically about half as long as the wire, since the wire circumscribes the table in a continuous loop.
The first step of the paper machine, with reference to FIG. 1, is called the formation. Water drains very quickly though the wire after the headbox. Within the first 20 feet of the wire table, much of the water content of the furnish has drained through the wire into the return trays, and the fibers and fillers have formed together in a layer of very wet paper. The last ⅓ of the wire table includes a series of vacuum boxes that sucks more water out of the furnish. A good even formation, with uniformity of the fibers and fillers is important and will result in a stronger, smoother sheet of paper.
At some point near the first or second vacuum box on the wire table, so much water has been drained or sucked away that the layer of furnish loses its watery shine and takes on a dull haze. The particular area on the wire table where shine turns to haze is commonly referred to as the wet line.
The haze that continues down the last part of the wire table is essentially an unpressed wet sheet of formed paper. At the end of the wire table the wet sheet of paper gets sucked off the wire fabric by a pick up roll and transferred onto the pickup wet press felt. The pick up felt is pressing lightly onto the wire fabric. The transfer of the wet sheet is achieved by the vacuum of the pickup roll turning inside of the pickup felt. The pressing section may include multiple presses.
The function of the wet pressing section of the paper machine is to press down the wet fibers and vacuum out more water from the wet sheet. Also, the wet press can to some degree correct or control the profile of the paper sheet with the use of hydraulic profile rolls in the pressing area.
After being pressed, the wet sheet is now transferred to the drying sections, where the wet sheet is carried by dryer felts over and around a series of dryer cans. After traveling over and around many very hot dryer cans, the paper has only 3 to 4 percent moisture or water content. At this point, the dry sheet is wound up at the end of the paper machine as a reel of paper.
A responsive and well-maintained guiding system is very important for efficient papermaking. A smooth, positive guiding system will properly guide the long wire, felt and fabrics in the center of the papermaking machine and will not let them oscillate from side to side.
The fundamentals for guiding any type of paper machine clothing (i.e. wire, felt, fabric, etc.) are basically the same. The felt always travels at right angles to the axis of the guide roll, as shown in FIGS. 2A and 2B. The guide roll generally is pivoted on the backside and movable on the front, and is actuated by a paddle member or palm guide that presses against the felt. This palm senses felt position, and by varying air pressure or mechanical leverage, automatically adjusts the movable side of the guide roll. The felt will move toward the side of the guide roll that it strikes first.
In the process of bumping up speed and efficiency of the paper machine, dryer sections became hooded. Hooded dryers evolved to become completely enclosed dryers. The air temperature inside modern enclosed dryer sections on the. big high speed paper machines can reach well over 300 degrees.
Linked with massive gears and individually weighing many tons, the dryer cans spinning inside the dryer sections are filled with steam temperature of over 400 degrees. Special high temperature monofilament dryer fabrics carry the wet paper over and around the hot dryer cans exceeding 4000 fl/minute. More speed and more heat has caused more problems. Stopping a section of a high speed paper machine can be compared to stopping a fast moving, fully loaded freight train, except the train has a braking system. When a fabric or felt guiding arrangement fails or malfunctions, the fabric or felt can shift its path. Quickly, the felt can run off and into the frame of the machine and become tangled. By the time a section of a huge paper machine can be stopped, the results of any guiding failure or malfunction can be devastating, destructive, and expensive. Guiding failures can be avoided with careful visual observation and routine preventative maintenance. However, the cause of many fabric or felt guiding failure or malfunctions is often the design engineering flaws incorporated into the original auto palm guide.
The auto palm guide is a constantly moving air bleedoff monitoring device controlling the steering action of the fabric or felt guide roll on a paper machine. However, currently available auto palm guides have not evolved enough to meet the demands of the modern high speed paper machine. The high speed paper machine is now in an environment of dust and dirt, 300 degree plus heat, and high moisture. This environment is the enemy of sealed bearings and unlubricated parts in constant movement, such as in the known auto palm guides.
Currently, two types of guiding arrangements are in use on high speed, fine paper machines. One commonly used auto palm guide is a single bleedoff type which utilizes a tapered metal plunger as a bleedoff monitor, usually with a single rubber diaphragm actuating the guiding arrangement. The single bleedoff type of auto palm guide is commonly considered disposable. The use of this tapered plunger device dates back to the early 1950""s and is still used as original equipment on machines made in the 1990""s. This design has many unlubricated parts and an average life expectancy of about two to six years.
An original single bleedoff tapered plunger type auto palm guide, shown for example in FIG. 3, is a simple device. This type of guide is relatively problem free, except for the metal plunger getting dirty and sticking at the xe2x80x9cpalm inxe2x80x9d position or the return spring breaking. The tapered plunger guide is considered disposable. The see-saw action of the plunger wears the round tapered hole of the plunger into an oval. The elongated hole will progressively leak more air.
After much use, the worn guide device can no longer build sufficient pressure in the guide diaphragm to push the guide roll out. Also at this point, the hole located at the top of the cast aluminum palm arm that connects the palm arm to the plunger will be worn excessively as will the palm arm pivotal bushings.
The second commonly used type of original equipment auto palm guide is a double bleedoff type, shown for example in FIGS. 4A-4D. The double bleedoff type is used with a twin bleedoff line diaphragm or a positioning cylinder type guiding arrangement. This type of auto palm guide is inherently more complicated, consisting of many more unlubricated moving parts. These parts include triple springs, twin ball and seats, adjustment pins, etc. Although double bleedoff type of devices are considered to be rebuildable, these units require frequent maintenance, and are unpredictable. With reference to FIG. 4, the reference numbers indicate the following components and parts: 1xe2x80x94air exhaust out; 2xe2x80x94stand; 3xe2x80x94fabric; 4xe2x80x94two air bleedoff inlets; 5xe2x80x94two fine springs; 6xe2x80x94rocker arm; 7xe2x80x94two O-ring seats; 8xe2x80x94two balls; 9xe2x80x94two palm arm bumpers; 10xe2x80x94two rocker arm adjustment pins; 11xe2x80x94air exhaust escape passage; 12xe2x80x94eye bolt; 13xe2x80x94palm arm; 14xe2x80x94main shaft; 15xe2x80x94two sealed bearings; and 16xe2x80x94palm arm return spring.
During a monthly maintenance shutdown, when the paper machine is stopped, it is not unusual to change out at least 4 of the 16 auto palm guides because they are not operating properly. Additionally, two common human errors can occur during installation. First, if the adjustable pins on the rocker cam are not set correctly after a rebuild, the double bleedoff guide will have either no free play or too much free play, causing oscillating or the loss of the fabric being guided.
Second, if the set screws on the pressure reducing (flow control) valve are not adjusted correctly, too little air flow will result in the positioning cylinder stalling in one direction. Alternatively, too much air flow stalls the palm arm in the center of its travel, because the palm arm return spring doesn""t have enough torque to push the balls out of their seats and lift the palm out. The difference between too little and too much air is about one sixteenth of a turn. If the positioning cylinder or the palm arm stalls, the fabric being guided can be ruined.
Admittedly, some human errors have been made in learning the quarks of the double bleedoff type guiding arrangement. For example, it would seem natural to turn the set screws on the pressure reducing (flow control) valve a quarter turn open to get more responsive action from the positioning cylinder. However, this is the most common mistake made by a novice. If too much air is provided, the palm guide will cease to operate and stall at any time without warning, resulting in a damaged fabric.
The following is a list of ten design flaws in the known double bleedoff guides that cause difficulties.
1) Eventually the O-rings on the pressure reducing (flow control) valve dry up and leak air. This results in sluggish guiding.
2) In time, the rubber strip palm arm bumper dries up and crumbles off. The palm arm then can travel so far that the rocker cam adjustment pin smashes into the balls seat and bends the pin. This wrecks the seat, causing guide failure or malfunction.
3) Often the palm arm return spring breaks from a stress fracture due to metal fatigue which results in guide failure.
4) Occasionally the area where the palm arm return spring is located accumulates enough moisture to rust the spring to pieces, resulting in guide failure
5) Occasionally the eye bolt on the palm arm return spring wears excessively. When this occurs two things can happen. Either the palm arm return spring loses some of its tension and can""t lift the paddle out, or the eye of the eye bolt gets so thin that the eye bolt breaks at the eye, both scenarios result in guide failure.
6) Often the fine springs that hold the balls in the seats wear thin and crack in half from rubbing on the walls of the air passages. With little or no spring pressure holding the ball in the seat, air pressure will not build and the positioning cylinder will stall. The result is guiding failure.
7) Often the (ball and seat) balls become egg-shaped or deformed from the normal operation of metal to metal contact of rocker arm adjustment pins pushing into to the balls. The balls then won""t seat which causes blow bye and oscillation of the fabric being guided.
8) Often rocker arm adjustment pins mushroom at the tips where they come in contact with the balls. This damage will shorten the life of the bails and seats, and also increase free play of the palm arm.
9) Often the O-ring seats dry up and shrink, causing blow bye, uneven pressure, and oscillation of the fabric being guided.
10) Very often the two small sealed roller bearings get wet, rust, seize up, or drag. Or their grease dries up and gets hard. Even high temperature bearings often fail. The roller bearings rotate the shaft to which the palm arm, palm arm return spring and rocker arm are fastened.
Also, the original equipment auto palm guides are primarily made of brass. Brass is an unstable metal for longevity in a paper machine environment with a 300 degree heat range. The expansion and contraction differential for brass is considerable. Also mixing dissimilar metals in a unit where close tolerances are required is not wise as the metals do not expand or contract together in parallel graduations.
The air switch and palm guide of the present invention is carefully and thoughtfully designed to bolt to or screw on most original auto palm guide supports. The air switch is engineered to mimic calibrated air bleedoff characteristics identical to that of the original equipment palm guide. The unique design of the radial air switch and palm guide allows it to work in conjunction with both single and double bleedoff style guiding arrangements. The radial air switch and palm guide of the present invention is made of all stainless steel except for the bearings, camshaft seal, Viton O-rings, and high temperature silicone palm arm bumpers. The palm arm is constructed of 2024 aircraft aluminum to reduce inertia for smoother and more constant contact to the fabric or felt. The only parts that can wear are the two inexpensive heavy duty roller bearings and the cam seal which are immersed in a constant flow of fresh oil and air.